This invention relates to a master cylinder having an indicator responsive to both a fluid level signal and differential pressure signal for informing an operator of a fluid condition in a brake system.
The U.S. Department of Transportation has proposed highway safety standards which require that the operator of a vehicle be informed of any adverse condition in a braking system which may affect braking.
Initially it was thought that a fluid level indicator, located in the reservoir of a master cylinder would be sufficient to meet the proposed safety standards. However upon reflection, it was determined that a failure in a brake supply line could occur and place an operator in a dangerous situation which would not be indicated until a low fluid condition occurred in the reservoir.
Thereafter, pressure differential warning devices, such as disclosed in U.S. Pat. No. 3,736,393 were introduced into brake systems of vehicles to monitor the fluid pressures supplied the wheel brakes. Such pressure differential warning devices are designed to activate a light on the dashboard of a vehicle whenever a predetermined pressure imbalance occurs between the hydraulic fluid pressures supplied the front and rear wheel brakes.
A light on the dashboard is usually connected to both the fluid level indicator and the pressure differential warning apparatus. Unfortunately, reed switches which are normally used in the fluid level indicators can stick and present a false fluid level signal to the warning light. In an effort to eliminate reed switches and their associated float actuators, it was suggested that the electrical capacitance of the fluid in the reservoir, which varies as a direct function of the volume of fluid in the reservoir be used to activate the light. However, it was discovered that the dielectric constant of currently available brake fluids vary with temperature, and produce false fluid level signals.